This invention relates to the thermal removal of binder from green bodies of ceramic particles and binder, more specifically to tolerable rates of green body temperature rise in combination with oxidizer concentration in a debinding environment of inert gas and oxidizer.
Ceramic structures are typically formed by initially combining a binder with ceramic particles or ceramic powder to form a malleable mixture. The mixture is shaped by one or more of a variety of processes, such as injection molding, extrusion, or tape casting into a green body. The green body is subsequently heated, usually in air, to drive out the binder and to sinter the ceramic particles together to form the desired structure. During the initial stages of heating, typically to approximately 800K, binder evolves from the body by thermal decomposition and evaporation.
The debinding phase of the heating is critical in the formation of sound ceramic structures. During the debinding phase, body damage such as bloating, internal pores or cracks may be produced by the internal pressure exerted by the evolving gas. The possibility of damage is increased by using submicron particles recently introduced for advanced ceramic components.
To avoid debinding damage to the body, low rates of body temperature rise are usually employed which result in undesirably long processing time. Often the debinding is conducted by moving bodies continuously through a furnace having a long heating path. However body temperature rise rate is difficult to control, and, particularly to vary in a selected schedule, during the course of travel of the bodies through such a furnace.
To reduce debinding time, several methods have been employed in the prior art. One approach disclosed in European Patent Office Publication Number 325,317 has been to continuously weigh a batch of bodies during heating in a furnace while adjusting the heating rate to keep the weight loss rate below a selected level which has been previously determined to yield undamaged sintered bodies.
Another method disclosed in U.S. Pat. No. 4,011,291 has been to pack the green body into a binder-absorptive material, and controllably heat the body above the melting point of the binder, but below the vaporization point of the binder. Binder is drawn from the green body by wicking action. Subsequently the body is fired.
Yet another method disclosed in U.S. Pat. No. 4,305,756 has been to heat green bodies a chamber wherein the pressure is raised above the vapor pressure of the binder in the green body at the temperature within the chamber. The latter three methods require costly apparatus, and being batch operations, suffer from low production rates. A debinding process is needed which is readily controllable, requires small initial investment in apparatus, has high rates of production and provides short debinding time.